Lokman Slim, a prominent Lebanese activist known for his opposition to Hezbollah, was found killed in his car in south Lebanon on Thursday.
Slim had not been heard from by his family since getting in his car on Wednesday evening to drive back to Beirut.
The body of the 58-year-old was slumped over on the passenger seat with multiple wounds from gunshots fired at close range, security and forensic officials said.
A journalist, political analyst and activist known as one of the leading Shiite voices criticizing Hezbollah, Slim was regularly attacked in media loyal to the group.
Slim's sister said before his death was even confirmed that his disappearance was inevitably linked to his opinions.
"He had a political stance, why else would he have been kidnapped," Rasha al-Ameer told AFP.
A relative said the family found out about his death from a news alert while at a police station to report his disappearance.
At the family home in the southern suburbs of Beirut, family members sat in silence and shock, while some wept.
Slim was often criticized by Hezbollah supporters for being instrumentalized by the United States.
The secular intellectual and pro-democracy activist had also made several documentary films with his wife Monika Borgmann, who had sounded the alarm on social media when her husband went missing.
France's Foreign Ministry called the killing of Slim a "heinous crime" and demanded a transparent investigation.
"France asks that the facts be clearly established and that all those who can contribute to establishing the truth contribute fully," Foreign Ministry spokeswoman Agnes von der Muhll said in a statement.
US Ambassador to Lebanon Dorothy Shea said Slim spoke publicly and privately about threats made against his life but continued his work, pushing for justice, accountability and rule of law in Lebanon. She called the killing an attack on Lebanon itself and urged a speedy inquiry.
“In a country that so desperately needs to recover from the multiple crises it faces, political assassinations send exactly the wrong signal to the world about what Lebanon stands for," she said.
UN Special Coordinator for Lebanon Jan Kubis also called for a quick investigation.
“This investigation must not follow the pattern of the Beirut port blast investigation that 6 months on remains inconclusive and without accountability. People must know the truth,” Kubis tweeted.
Amnesty International's deputy director in the region, Lynn Maalouf, said Slim was the “victim of this decades-old pattern of impunity which has ensured that past and present targeted killings of activists, journalists and intellectuals remain unpunished, and for which the Lebanese state is ultimately responsible.”
She also called on Lebanon to ensure a transparent investigation into the killing.
